My first serious effort at star photos.

[ameerat42]

Mr Felix. Don't flog yourself too much. 1.5 minutes where Orion stalks, near the celestial equator, is WAY to much exposure time for almost ANY lens that is not tracking the subject. OF COURSE, though I couldn't be sure, it would be a FAIR BET (you wanna bet?) that what you posted is NOT camera shake but star trailing. Think of Occam's Razor. Why invoke the less probable when the simplest explanation is to hand?

Yes, your focal length was 18mm, but as I said in the earlier post, I had appreciable trailing at 15 sec with 24 mm. Do the trigonometry: 360degrees in 24 hours is 15 degrees per minute, Over a focal length of 18 mm, 75sec (EXIF) is about 0.1mm on your sensor.

Given that it is an approx 1.5 crop sensor, that roughly translates to the trailing you have shown.

ONE Question: Is f/6.3 your widest aperture at 18mm? If you can go wider, ie, go to maximum aperture, and use an ISO of about 800-1600. Do a bit of noise reduction on the result, maybe. Keep it up, though. It's a challenge.
Am.

PS: Just read your last point. You do not "calculate" exposure times for stars. Bulb, widest aperture, most exposure time you can squeeze in before trailing is expected. Meantime, (Much later on summer nights) give the Southern Cross and Pointers a whirl. They are nearer the S celestial pole and their RADIAL motion is less than stars nearer to Orion. (Their angular motion is, of course, the same.)